Many of us have found out that car phone USB chargers are a source of broadband noise. I have even heard cars drive past and get a swoosh of radio noise when they do.But as more vehicles have them built-in and hard wired, is the problem going to get worse?

The simple answer is to not use them while listening, but are there quiet ones out there? If not- would it be worthwhile making one? Maybe we can trade efficiency for quiet audio, but I have designed power supplies with enough filtering to remove this noise, and during research on that found a few ic's that incorporate jitter to reduce specific frequency noise.

I have a couple that fit in the cigarette socket that are terrible, so was thinking of a hard-wired alternative that is switched - and quiet.

Problem seems to be ringing from a very cheap buck converter design that doesn't even use a SMD shielded inductor, uses axial leaded switching diode, and no filter caps at all on the 12v input.

There is a module out there that is called a KIS-3R33S that is a well designed board mount module. It is however configured for 3.3v output but by altering the SMD resistor network in there (or removing it and making your own on a motherboard) you can make it output what you like, and with a bit of extra filtering gets the noise down to 20mV. It also uses a shielded inductor, schottky diodes and most importantly - the whole thing is inside an RF shielded plastic case.

So the Kis module turned up so time for some testing on the bench. Using a filtered 13.8v supply and a cheap portable shortwave radio for detection.I can pick up noise from CCFL lights from 4 feet away with it so is good for a general test.

The Lil round single charger was noisy, but the square double outlet one was absolutely Horrible. The lil round one stayed on frequency but the square one would vary it's frequency quite a bit up and down the band seemingly at random. It was not stable at all.

Both were sending quite a lot of noise back up the supply, and for the square one this was worse.

The kis module was very quiet, and had to get as close as 4" to pick up any hiss from it, compared to several feet for the other two

Opening up all 3 the reasons are apparent. The round one had an axial diode as part of the resonant circuit so would just radiate emf from that, and both used bobbin style open inductors. The round one did at least have a 22uf cap on the input and a 100uf cap on the output, but the leads for that had bad solder joints. The square one only had small tantalum smd caps, but did have holes for larger electrolytics - that were not fitted.

The kis module had a fully shielded inductor, good smd board layout, and most importantly has a shielded box that fully encloses it above and below the board. The module as supplied also had a 1000uf cap on the output which helps greatly, but nothing on the input. There already may be an effective filter network on the board though.

I did try and improve the square charger by adding a filter cap to the output, but upon testing it was putting out 13.8v, which is a bad sign. Either I overheated it during soldering it on, or some other error I have not seen, but it did not like it, so is now in the scrap pile.

I am now looking to make the kis module more permanent in my vehicle in a metal box with on off switch, and put a double socket on the output. Maybe then I can listen to radio and charge my phone!

Richie_asg1 wrote:Many of us have found out that car phone USB chargers are a source of broadband noise. I have even heard cars drive past and get a swoosh of radio noise when they do.

I have heard such noise from passing cars (smothering weaker FM stations), I suspect it was due to the power supplies of cheap aftermarket LED headlights, it was when these headlights on the passing cars were on.